The mouse embryo chimera assay is a very sensitive test for detecting in vivo exposure of the mammalian germ cell to ionizing radiation. This exposure produces a competitive embryonic cell proliferation disadvantage that becomes expressed by the embryo from the exposed parent when challenged by cell-cell contact with a control embryo in an aggregation chimera. The detection limits of this assay for low-LET ionizing radiation (gamma or x rays) are between 0.005 Gy and 0.01 Gy for parental male irradiation and between 0.05 Gy and 0.15 Gy for parental female irradiation. During the current funding period we were successful in achieving our major objective, which was to determine whether the radiosensitive target for the assay was nuclear or extranuclear. The evidence we obtained strongly favors the hypothesis for a nuclear radiosensitive target in this competitive renewal application, we now wish to test hypotheses regarding the possible consequences of damage to this nuclear target and the predictive value of the chimera assay for transmitted embryonic effects using nuclear endpoints. Specific Aim 1 will test the hypothesis that this nuclear target consists of genes that affect cell proliferation rate through their impact on the efficiency of growth factor/receptor autocrine "loops". This hypothesis is based upon the known expression of several functional autocrine loops by the early embryo and on the accumulating evidence that growth factors, their receptors, associated signal transduction components and transcription factors are radiosensitive in a wide variety of cell lines and tissues. Specific Aims 2 and 3 are concerned with the predictive value of the chimera assay for embryonic effects resulting from parental exposures to ionizing radiation. As is the case for Specific Aim 1, the hypotheses tested here are predicated upon a nuclear radiosensitive target for the chimera assay. Specific Aim 2 will test the hypothesis that higher doses to the female germ line (0.3 Gy - 0.5 Gy of 137 Cs gamma radiation) will result in a temporal correlation between positive results of the chimera assay and the stages of oogenesis that transmit increases in developmental anomalies as reported in the literature (e.g., the specific locus test). Specific Aim 3 will test the hypothesis that there will be temporal correlations between positive results from the chimera assay and increases in frequencies of chromosomal aberrations detected in their germ cells by fluorescence in situ hybridization in males.